Abrasives



United States ABRASIVES John RfGregor and Harold E. Van Orden, Tifiin,Ohio, assignors to Abrasive & Metal Products Co., Detroit, Mich., acorporation of Michigan No Drawing. Filed Mar. 11, 1951, Ser. No.644,974.

8 Claims. c1. 51-298) atent In the production of abrasives a mixture ofabrasive ing of the surface of the abrasive, and to otherwise improvecutting efficiency. The filler material usually employed is a finelyground or powdered natural mineral known as cryolite, having the generalformula Na AlF Even though cryolite significantly improves the cuttingefiiciency of grinding wheels, in the grinding of stainless and alloysteels the performance of cryolite-filled grinding Wheels has leftsomething to be desired. Moreover, since. cryolite is largely imported,its supply and price are subject to wide fluctuations in times ofinternational crisis.

It is an object of this invention, therefore, to provide an abrasivearticle which contains an inexpensive, readily-availablefiller orfillers. and whichhas an efiiciency at least equal to that of cryolite.Another object is to provide a filled abrasive article which Will moresatisfactorily grind stainless steel and other tough alloy steels. Stillotherobjects and advan- 'ta ges of the presentinvcntion will beapparent, or will become apparent, in the more specific description ofthe invention to follow.

In accordance with-the present invention an abrasive article is providedcontaining at least one filler selected from the class consisting of thealkali metal salts of sulfuric, hydrochloric and hydrobroinic acid andmixtures of at least one of the latter salts with lead sulfide. Most ofthese fillers or combinations of fillers are inexpensive and readilyavailable in large quantities at all times. With combinationsof thealkali metal salts and lead sulfide in certain proportions, synergisticeffects are noted, that is, the combination is much more efiective thaneither of its components when used alone and an abrasive article isproduced which is capable of cutting faster and which has longer lifethan heretofore known comparable abrasive articles manufactured forsimilar purposes. The abrasive articles of this invention are mosteffective in the cutting or grinding of stainless and alloy steels, tWhile we do not wish to be bound by theory due to the fact that thefunction of fillers is imperfectly understood, the following explanationis offered only because it appears to fit the facts as presently known.The stainless alloy steels and various of the other alloy steels high intotal alloying ingredients, are characterized by .iplighness andabrasive resistance. Consequently,

armingpressures and/or grinding times, must be inmaterial.

2,939,777 Patented June 7, 1960 creased to remove a given Weight ofmetal, as compared to iron or mild steels. Consequently, the abrasivewheels wear away at an increased rate and temperatures generated at themetal-to-abrasive interface will usually be higher and may, in fact, beso high that a thin surface layer of the metal itself may be burned,oxidized, or otherwise heat-modified to make it unsightly. The hightemperatures may cause the heated metal to load the wheel and reduce itscutting efficiency. Likewise, the bond material, and particularly thefiller content thereof, may be modified, volatized or otherwise renderedinefiective at the temperatures obtained. We have observed that manymaterials which have melting points above about 1200 C. are noteffective as fillers in abrasives for grinding the alloy steels. Thismight indicate that a good filler has a fluxing, protective or otherfunction and that such function requires a liquefaction of the filler.Further support for the latter surmise is furnished by the observationthat many of the most effective mixtures or combinations of fillers ofthe invention either are at or near their eutectic composition, or arecompositions exhibiting lower melting points than one or both of itscomponents used alone. It is believed that the sulfur of the sulfides isdissolved by the molten alkali metal salts to form a low melting fluxingcomposition that greatly facilitates grinding.

The abrasive articles of this invention may be any molded abrasive madefrom a mixture including abrasive grains and a bond material containingone or more of the above fillers and a resinous thermosetting binder Theabrasive articles of this invention which are particularly effective aremolded abrasives having a high density, that is, an abrasive articlehaving a low percentage of void spaces (i.e. less than 15%). It is thelatter type that is utilized in heavy duty grinding such as snagging.Any abrasive grain or mixture of grains may be employed includingsilicon carbide, boron carbide, tantalum carbide, tungsten carbide orother hard metal carbides; alumina such as emery, and including electricfurnace fused alumina such as corundum, diamond grains, glass, quartz,garnet, etc.

The resinous binder material in the bond material must be curable,heat-hardenable or heatconvertible to form a hard, strong bond whichwill not readily deteriorate at elevated temperatures. An illustrativebinder resin found particularly satisfactory in the abrasives of thisinvention is the fusible B-stage phenolic resins (phenol-aldehyde orcresol-aldehyde resins which are available in either powdered or liquidform. If desired, the heat-convertible binder resin can be modified withsmall proportions of other resinous or rubbery materials such as theepoxy resins, vinyl resins including those of vinyl chloride, vinylbutyral and others, styreneacryl-onitrile resins,butadiene-acrylonitrile rubbers, and others and may contain varyingproportions of crosslinking or curing agents such as hexamethylenetetrarnine, paraformaldehyde, and others. In some cases solvents orplasticizers such as furfuraldehyde, propylene sulfite,

fu=rfuryl alcohol, cresol, and others may be presentinthe bond mix, ifdesired. The bond mix may also contain, in addition to the fillers ofthis invention, any of the conventional fillers and other additivematerials such as cryolite, feldspar, iron oxide, and others. Ifdesired, the bond may also contain lime, and where furfuraldehyde isemployed, the presence of limeis recommended.

The mixture of this invention which is molded may contain any operableratio of abrasive to bond although, in general, for heavy duty grindingwheels the mixture will usually contain from about 54% to about 64% byvolume of abrasive and from about 36% to about 46% by volume of thebonding material, it beingunderstoojd that the bonding material sospecified includes the binder resin, fillers, plasticizers and otheradded non-abrasive ingredients. illustrative abrasive wheel mixture maycontain from about 54% to about 64% by volume of abrasive, from about1.6% to about 26% by volume of binder resin or resins, from about 1% .toabout 4% b y volume of lime, from about .'8'% ,to about 20% of totalfiller,.and from '0 to200 ml. of furfuraldehyde forevery pound ofpowderedphenol-aldehyde resin, if the latter type isemployed.

The aboveproportions used in ,theabras'ives ofthis invention relatetozthe total volume of the finished abrasive. The proportion of filler,as related to the bonding material, can also vary considerably depending.on the use ,to be made of the finished abrasive. Generally, the totalfiller content of the bonding material should range from about to about40% by volume, more preferably from about '-10%-.to about 35%. Theseproportions include both the fillers of this invention and any otherconventional filler or fillers which may be present. In general, anyfiller in addition to the filler of this invention should constitute nomore than half the total filler and in no case should the proportion ofthe ,fillers of the present invention constitute less than about 2% byvolume of the total bonding material.

In combination with lead sulfide, any ,of the alkali metal salts ofsulfuric, hydrochloric and hydrobromic acids may be employed as fillersin the abrasives of this invention including those of lithium,-sodium,potassium, rubidium and cesium. Illustrative salts of this type includesodium sulfate, potassium sulfate, lithium sulfate, cesium sulfate (Cs Qrubidium sulfate (Rb 'SO sodium bromide, sodium chloride, potassiumbromide, potassium chloride, and many others. All of the known alkalimetal salts of these acids melt at temperatures below about 1100 C. andmany of the more common salts of this type melt at temperatures between400 C. to about 900 C. Of these, the sodium and potassium sulfates,sodium and potassium chlorides, and sodium and potassium bromides aremost readily available, are less expensive and have been found to .bevery efiicientfillers when used in admixture With'lead sulfide.

Whenlead sulfide is added to the abrasive along with one or more of thealkali metal salts, a great improvement in the cutting efiiciency of theresulting abrasive is usually noted. This is surprising since leadsulfide, when used as a sole filler, produces abrasives which have acutting efiiciency that is not significantly better than cryolitecontaining abrasives. As little as 5% lead sulfide by volume, based onthe total volume of alkali metal and lead sulfide 'filler, significantlyimproves cutting efliciency. In most cases, as the proportion of sulfideis increased, large increases in cutting efficiency are realized until apoint is reached Where further increases in cutting effioiency are notobtained. For example, with mixtures of lead sulfide and sodium bromide,sodium chloride, potassium chloride, potassium bromide and sodiumsulfate, the maximum cutting efiiciency is obtained with mixtures ofabout 75% lead sulfide and 25% of the salt by volume based on thetotal'volume of alkali metal and lead sulfide fillers. The efiiciency ofthe various salts at the ratio of 75% lead sulfide,'25% of the salt isin the order named. When potassium sulfateis used in combination withlead sulfide, the maximum cutting e'fficiency occurs at mixtures 0f25%lead sulfide and 75 potassium sulfate, but the cutting efficiency dropsoff only slightly as the proportion of'lead sulfide is increased to 75%lead sulfide and 25% potassiumsulfate. Increases in lead sulfide aboveabout 75%, with the balance of the filler being one of the salts of theabove named acids, decreases the cutting efficiency relatively sharply.The cutting efficiency of the admixed filler is'better than thatobtainable with .cryolite .even when the mixtures contain 90% leadsulfide and of theselected salt. .,Statedin terms of ratio, abrasives.having snagging grinding characteristics superior to that obtainablewith cryolite can Zbe'made with the ratio of the 4 lead sulfide toalkali metal salt within the range of 9:1 to about 1:8.

The abrasive products of this invention can be made in any conventionalmanner. For example, the conventional coldor hot-pressing methods may beemployed wherein the abrasive grains are first wet with furfuraldehydeor another plastioizer or solvent and the binder resin, fillers, andother dry, powdery .orliquid ingredients then added to the wetted grainsand mixed until homogeneous. .The resulting :darnp mixture then isplaced in either a hot or cold mold and pressed under high pressure toform a coherent green abrasive which is removed from the mold andheatedin anoven to cure the bond. This displacement method disclosed inthe copending application of John R. Gregor and Samuel L. Kistler,Serial No. 499,443, and now Patent No. 2,860,961, filed April 5, 1955,may also be employed to advantage. In the latter method,dry,.loose;abrasive grains are placed in a suitable mold anda continuouslayer of ailuid or .fluidized bond mix applied to one exposed surface ofthe grain layer in the mold. Air is removed or is permitted toescapefromthe surface ,of the :grain layer opposite the continuous bond layer toproduce a flooding, directional movement of the bond layer toward theopposite surface to ensure completefilling of the voids. Gravity,vacuum, gas pressure, orcentrifugal force may be applied to one side orthe other of theabrasive layer to .increase the speed of the directionalmovement of the bond. .The abrasive is then baked .to cure the .bond..Any other method may be employed which is capable of producing a sound,.com' pact, well-bonded abrasive article.

The invention will now be described ,in greater detail in connectionwith .a number of illustrative specificexamples. Unless otherwisespecified in the examples, ,all proportions will be expressed as byvolume.

A plurality of series of abrasive wheels were made by the ,cold pressingtechnique using a mixture containing 60% by volume of abrasiveandapproximate1y..3 8% to 40% .by volume of bonding material includingplasticizer.

To the dry abrasivegrains there was first added cc. of furfuraldehydefor every pound of binder resin tobe employed. The liquid and abrasivewere then mixed until a uniform consistency was obtained. The powderedbonding materials were first combined with '30 cc. of the equivalent ofa neutral tar oil before being added to the somewhat moist mixture ofabrasive and furfuraldehyde. Mixing was then resumed until a homogeneousmixture was .obtained. Wheels were made from this mixture'by coldpressing.

Each series of wheels included four control wheels which containedpowdered cryolite as the sole filler. Four other control wheels weremade employing lead sulfide as the sole filler and four additionalfillers were made employing the selected alkali metal salt as the solefilled. Four wheels were made containing a mixture of lead sulfide andthe selected alkali metal salt, constituting the entire filler, in theproportions of 75 alkali metal salt/25 lead sulfide, 50 alkali metalsalt/5.0 lead sulfide, and 25 alkali metal salt/ 75 lead sulfide byvolume. volume of'the finished wheel. Each of these wheels, .aftercuring, was then employed on a standard abrasive wheel grinding testmachine in the grinding of Type 30.2;stainless steel. Each wheel wasground for waperiod 01f :five minutesat the ,samepressure of about250'lbs. ,per,square inch, which is typical of pressures commercially:em-

ployed in such ,grinding operations. The data presented In all cases thetotal filler constituted 27% by sults obtained with the four wheels.

PbS with K 80 Steel Filler removal S/W Q lbs/hr.

PbS with KCl Wheel Steel Filler wear removal S/W Q mfi/hr lbs/hrCryollte 58 7 59 93. Cl 04 614 S0. 9 75 KGl/25 PbS. 72 .828 110.3 501101/50 PbS 90 .908 118. 25 KCl/75 PbS 74 .951 125. 5 H38 88 784 99. 3

Wheel Steel Filler Wear removal S/W Q infi/hr lbs/hr.

54. 78 41. 58 .759 93. O 75. 65 49. 44 654 88. O 60. 41 51 60 .854 111.6 55. 14 51.48 .934 120. l 47. 21 47. 22 l. 000 126. 5 57. 21 44. 88 78499. 3

PbS with KBr Wheel Steel Filler wear removal S/W Q lnfi/hr. lbs/hr.

S/W Q The column headed Q represents a numerical evaluation of theover-all quality of the wheel and gives the Q number or quality number.The value of the quality number Q is expressed in the equation where Wis the loss in weight of the wheel in cubic inches per hour, S is thepounds of steel or metal removed per hour and C C and K are constants.The value of these constants was determined by plotting the results ofseveral hundred grinding tests using the conditions above specified withcryolite filled wheels and calculating the value of the constants fromthe plotted line. The constants C C and K so determined are 5549, 6000and 136, respectively. The above Q values were all calculated usingthese constants.

It may be seen from an inspection of the above examples that thecombination of lead sulfide with each of the alkali metal saltsillustrated gives S/ W values and Q values which are greater thancorresponding values for cryolite alone, or lead sulfide alone, or thealkali metal salt alone. In making comparative grinding tests, manyvariables may affect the actual results obtained and in order to obtainreliable comparison figures it is necessary to utilize wheels which havebeen manufactured on the same day and cured at the same time or underidentical curing temperature conditions. Moreover it is necessary to doall of the grinding of any one test on steel obtained from one batch. Inall of the above tests these conditions were followed so that therelative figures Within a specific test are meaningful but it should beunderstood that because of the variations which may occur in the steelbeing ground, the conditions of wheel manufacture or the conditions or"operation of the testing equipment, it is not reliable to drawconclusions by comparing one set of test data against another. Forexample, in the above data relatively wide numerical variations existbetween the S/ W ratios for the lead sulfide control wheels employed inthe potassium bromide and sodium bromide tests relative to similarwheels employed in the other tests and yet the data is reliable, on acomparison basis, in its own particular test.

What is claimed is:

1. An abrasive article comprising abrasive grains and a bondingmaterial, said bonding material comprising a thermoset binder resin anda filler, at least /z of said filler consisting of at least one alkalimetal salt of sulfuric, hydrochloric and hydrobromic acids in admixturewith lead sulfide, said admixed filler constituting at least about 2% byvolume of the total bonding material, the ratio of lead sulfide to saidalkali metal salt being within the range or" about 9:1 to about 1:8.

2. An abrasive article in accordance with claim 1 wherein said admixedfiller consists of potassium sulfate and lead sulfide.

3. An abrasive article in accordance with claim 1 wherein said admixedfiller consists of potassium chloride and lead sulfide.

4. An abrasive article in accordance with claim 1 wherein said admixedfiller consists of potassium bromide and lead sulfide.

5. An abrasive article in accordance with claim 1 wherein said admixedfiller consists of sodium bromide and lead sulfide.

6. An abrasive article in accordance with claim 1 wherein said admixedfiller consists of sodium chloride and lead sulfide.

7. A cured molded abrasive article comprising abrasive grains, athermoset phenolic binder resin and a filler consisting essentially of amixture of lead sulfide and at least one alkali metal salt of sulfuric,hydrochloric and hydrobromic acids in which the ratio of lead sulfide tosaid alkali metal salt is within the range of about 9:1 to about 1:8,said admixed filler constituting at least about 2% by volume of thetotal bonding material.

8. A cured molded abrasive article comprising abrasive grains, athermoset phenolic binder resin and a filler consisting essentially of amixture of lead sulfide and at least one alkali metal salt of sulfuric,hydrochloric and hydrobromic acids and in which said lead sulfide ispresent in an amount of from about 5% to about by volume of the totalvolume of said filler, said filler constituting at least about 2% byvolume of the total bonding material.

Retercnces Cited in the file of this patent UNITED STATES PATENTS2,216,135 lainer Oct. 1, 1940 2,325,180 Egeberg July 27, 1943 2,638,413Robie May 12, 1953 2,811,430 Gregor et a1 Oct. 29, 1957

8. A CURED MOLDED ABRASIVE ARTICLE COMPRISING ABRASIVE GRAINS, ATHERMOSET PHENOLIC BINDER RESIN AND A FILLER CONSISTING ESSENTIALLY OF AMIXTURE OF LEAD SULFIDE AND AT LEAST ONE ALKALI METAL SALT OF SULFURIC,HYDROCHLORIC AND HYDROBROMIC ACIDS AND IN WHICH SAID LEAD SULFIDE ISPRESENT IN AN AMOUNT OF FROM ABOUT 5% TO ABOUT 90% BY VOLUME OF THETOTAL VOLUME OF SAID FILLER, SAID FILLER CONSTITUTING AT LEAST ABOUT 2%BY VOLUME OF THE TOTAL BONDING MATERIAL.